There will still be one Dan Sullivan on the ballot, but election officials in Alaska determined a second man by the same name cannot run against him in the high-stakes Senate race.

A man named Dan Sullivan, or Daniel J Sullivan Jr, filed to run as a Republican against incumbent Alaska senator Dan S Sullivan, also a Republican. Republicans filed complaints against the other Dan Sullivan, saying the candidate had coordinated with a Democratic campaign to confuse voters.

Alaska’s US Senate election is seen as competitive and is a key target for Democrats hoping to win back control of the upper chamber. Mary Peltola, a former Democratic congresswoman, is expected to face incumbent Sullivan in November.

The state has a non-partisan primary, set for 18 August. The top four vote-getters advance to the general election, which uses ranked-choice voting.

In a letter to Daniel J Sullivan on Monday, Alaska elections director Carol Beecher wrote that the “utterly unprecedented facts” in the case led her to conclude that Dan J Sullivan’s declaration of candidacy for US Senate “was not filed in order to declare an actual good-faith candidacy for the office of United States senator, but was instead filed with a purpose to confuse or mislead and to thereby compromise the ballot’s fairness or neutrality”.

A review of Sullivan’s candidacy raised a series of concerns, Beecher wrote: he did not typically use the nickname “Dan”; he had recently registered as a Republican; his campaign website looked like the other Dan Sullivan’s; and his campaign consultant was a longtime Democratic party supporter, including of Peltola.

“I conclude that the preponderance of the evidence is that you chose this new nickname and party affiliation because that name and party affiliation happen to be the name and party affiliation of another candidate in the race,” Beecher wrote.

Beecher is a Republican, and her office is overseen by Republican lieutenant governor Nancy Dahlstrom.

Dan J Sullivan, a former teacher, previously told the New York Times that election officials were seeking to “protect an incumbent senator from facing competition at the ballot box” and said he had not coordinated with Peltola. Peltola’s campaign has denied coordinating with Sullivan.

In a previous response to questions from Beecher’s office, Dan J Sullivan said the National Republican Senatorial Committee was using “your office as a pawn to kick me off the ballot”. He said he has long gone by Dan, registered as a Republican after his previous political party had disbanded, and includes photos of himself on his campaign website, adding that he looks nothing like the more prominent Dan Sullivan.

“I am a qualified candidate who followed the rules and filed to run for office under my legal name,” Sullivan, the Senate hopeful, said in a campaign statement last week.

He added: “The people of Alaska are fully capable of deciding for themselves who should represent them in Washington. If Senator Sullivan believes he has served Alaska well during his 12 years in office, he should make that case directly to voters. He should not rely on government officials or legal maneuvers to limit who can challenge him.”

Dan J Sullivan can challenge the determination in court to try to appear on the ballot, Beecher noted, though primary ballots are set to print on 28 June.

The Republican National Committee called Dan J Sullivan “Decoy Dan” and said his removal from the ballot was a win for election integrity.

“Alaskans can rest easy that their leaders will never tolerate blatant attempts to mislead voters and rig elections like Democrats’ Decoy Dan Scam,” RNC chairman Joe Gruters said in a statement. “This is the right decision that will protect Alaskans from an unprecedented attack on our democracy.”